1. Technical Field
This invention relates to the science of initiating chemical conversions by catalysts, and more particularly to the technology of reducing the light-off temperature to improve catalyst efficiency.
2. Discussion of the Prior Art
Conventional platinum automotive exhaust catalysts have a propane light-off temperature of about 495.degree. C. Light-off temperature means the temperature at which a 50% conversion efficiency is reached with respect to the gas being converted. It is desirable to achieve lower light-off temperature for hydrocarbon oxidation because hydrocarbon emissions during cold-start operation of an automobile can be significant and, if untreated, can significantly affect the emission content that is released. For example, during the first 60 seconds of cold-start, 65-70% of hydrocarbon emissions remain unconverted.
The prior art has not recognized that titania can lower the light-off temperature for hydrocarbon when (i) used in concert with platinum and (ii) applied in a certain manner. Heretofore, titania has been used as a continuous washcoat over platinum to impart resistance to SO.sub.2 (see U.S. Pat. No. 4,650,782), or as a more stable base on Al.sub.2 O.sub.3 for overcoating platinum (see U S. Pat. Nos. 2,894,901; 4,219,447; 4,237,030; 4,134,860; and 4,478,797).
One of the applicants herein has previously disclosed, in copending U.S. Ser. No. 440,525, filed Nov. 22, 1989, now U.S. Pat. No. 5,102,853, commonly assigned to the assignee herein, how a palladium/alumina catalyst can be modified by application of organo-titanates to increase high temperature conversion efficiency. However, differing chemical complexing abilities of precious metals (which result from differing electronegativities and differing thermodynamic characteristics of reactions involving the precious metal) cause researchers to believe such success is highly selective to palladium. For example, palladium is more reactive to O.sub.2 than platinum. Thus, the direction of the prior art would not make it obvious to realize that organo-titanates might have a low temperature conversion efficiency improvement as well as enhancing sulfur poisoning resistance. It would be reasonable to assume that platinum would not react the same with organo-titanates and not only would not improve high temperature conversion efficiency but would fail also to enhance low temperature conversion efficiency.
What is needed is an oxidation catalyst that is more effective to provide hydrocarbon conversion at light-off temperatures as low as 150.degree. C.